In a wireless communication system, e.g. 3G, IEEE802.16d, IEEE802.16e, ranging is needed to maintain the synchronization of uplinks. FIG. 1 is a schematic diagram illustrating a frame structure of IEEE802.16d or IEEE802.16e. As shown in FIG. 1, in a conventional ranging method, uplink ranging and uplink services share the same segment of time slots, but respectively occupy different sub-channels. After study, the inventor finds that the conventional ranging method has the following problems.
Before the ranging is finished, because original synchronization between a base station (BS) and a user terminal (UT) is not so ideal and closed loop power control is not started, the UT usually sends uplink ranging messages by relatively large transmission power, and thus a ranging channel will bring relatively large interference to a service channel. Especially, when a service BS suddenly restarts, all UTs under the service BS will send uplink ranging messages in a short time, and thus numerous ranging signals will bring great interference to the service channel, thereby badly interfering normal communication of the service channel.
FIG. 2 is a schematic diagram illustrating a conventional ranging problem in a wireless communication system. As shown in FIG. 2, a base station A (BS_A) and a base station B (BS_B) are two adjacent co-frequency BSs. There are a terminal ‘a’ (UT_a) and a terminal ‘b’ (UT_b) under the BS_A. In the same time slot, the UT_a performs uplink ranging and the UT_b performs uplink service communication. The UT_a does not keep synchronization with the BS_A and has relatively large transmitting power, thus normal uplink services of the UT_b are disturbed badly.
A BS may receive a downlink signal from a remote co-frequency BS sometimes, and the downlink signal may bring interference to ranging signals of the BS. The interference may be very large sometimes and even may result in that UTs under the BS can not access the BS, and thus services of the BS are badly influenced.
Also as shown in FIG. 2, the BS_A and a base station C (BS_C) are two remote co-frequency BSs. If a time period that a downlink signal of the BS_C arrives at the BS_A partly overlaps a time period that a ranging signal of the UT_a arrives at the BS_A, the downlink signal of the BS_C may bring interference to the ranging signal.